Monolithic integration of heterojunction solar cells

ABSTRACT

A method for fabricating a device with integrated photovoltaic cells includes supporting a semiconductor substrate on a first handle substrate and doping the semiconductor substrate to form doped alternating regions with opposite conductivity. A doped layer is formed over a first side the semiconductor substrate. A conductive material is patterned over the doped layer to form conductive islands such that the conductive islands are aligned with the alternating regions to define a plurality of photovoltaic cells connected in series on a monolithic structure.

RELATED APPLICATION INFORMATION

This application is a Continuation application of co-pending U.S. patentapplication Ser. No. 14/796,546 filed on Jul. 10, 2015, which is aDivisional application of co-pending U.S. patent application Ser. No.13/748,672 filed on Jan. 24, 2013, which is a Continuation applicationof co-pending U.S. patent application Ser. No. 13/718,819 filed on Dec.18, 2012, all of which are incorporated herein by reference in theirentirety.

BACKGROUND Technical Field

The present invention relates to photovoltaic devices and fabricationmethods, and more particularly to monolithic integration of photovoltaiccells in a semiconductor substrate.

Description of the Related Art

Monolithic integration of solar cells is desirable for realizinghigh-voltage power supplies for various applications such as powering upa chip or autonomous circuit operation. Heterojunction solar cells areof interest for this purpose, generally due to their high-efficiencyand, more particularly, heterojunction solar cells result in higher opencircuit voltages than homojunction solar cells at comparable conversionefficiency values.

SUMMARY

A method for fabricating a device with integrated photovoltaic cellsincludes supporting a semiconductor substrate on a first handlesubstrate and doping the semiconductor substrate to form alternatingregions with opposite conductivity. A doped layer is formed over a firstside the semiconductor substrate. A conductive material is patternedover the doped layer to form conductive islands such that the conductiveislands are aligned with the alternating regions to define a pluralityof photovoltaic cells connected in series on a monolithic structure.

Another method for fabricating a device with integrated photovoltaiccells includes doping a semiconductor layer on a buried dielectric layerof a semiconductor-on-insulator substrate to form alternating regions ofhigher and lower dopant concentration across the semiconductor layer;forming at least one doped layer over a first side the semiconductorlayer; and patterning a conductive material over the least one dopedlayer to form conductive islands such that the conductive islands arealigned with the alternating regions of high and low dopingconcentration to define a plurality of photovoltaic cells connected inseries on a monolithic structure.

Yet another method for fabricating a device with integrated photovoltaiccells includes doping a semiconductor layer on a buried dielectric layerof a semiconductor-on-insulator substrate to form alternating regions ofhigher and lower dopant concentration across the semiconductor layer;forming at least one doped layer semiconductor over a first side thesemiconductor layer, the at least one doped layer including ahydrogenated crystalline semiconductor material; patterning a conductivematerial over the least one doped layer to form conductive islands suchthat the conductive islands are aligned with the alternating regions ofhigh and low doping concentration to define a plurality of photovoltaiccells connected in series on a monolithic structure; and etching the atleast one doped layer using the conductive islands as an etch mask tocreate spacings between portions of the at least one doped layercorresponding with each of the plurality of photovoltaic cells.

A device with integrated photovoltaic cells includes a semiconductorsubstrate including doped alternating regions with opposite conductivityand at least one doped layer formed over a first side the semiconductorsubstrate. A patterned conductive material is formed over the least onedoped layer forming conductive islands such that the conductive islandsare aligned with the alternating regions to define a plurality ofphotovoltaic cells connected in series on a monolithic structure.

Another device with integrated photovoltaic cells includes asemiconductor-on-insulator substrate having a base semiconductor, thinsemiconductor layer and a buried dielectric layer therebetween, the thinsemiconductor layer having alternating regions of higher and lowerdopant concentration across the thin semiconductor layer. At least onedoped layer is formed over a first side the semiconductor layer, and apatterned conductive material is formed over the least one doped layerforming conductive islands such that the conductive islands are alignedwith the alternating regions to define a plurality of photovoltaic cellsconnected in series on a monolithic structure.

Yet another device with integrated photovoltaic cells includes asemiconductor-on-insulator substrate having a base semiconductor, thinsemiconductor layer and a buried dielectric layer therebetween, the thinsemiconductor layer having alternating regions of higher and lowerdopant concentration across the thin semiconductor layer. At least onedoped layer is formed over a first side the thin semiconductor layer.The at least one doped layer includes a crystalline semiconductormaterial. A patterned conductive material is formed over the least onedoped layer forming conductive islands such that the conductive islandsare aligned with the alternating regions of high and low dopingconcentration to define a plurality of photovoltaic cells connected inseries on a monolithic structure. Spacings are formed between portionsof the at least one doped layer corresponding with each of the pluralityof photovoltaic cells.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1A is a cross-sectional view of a device having a plurality ofphotovoltaic cells monolithically integrated and connected in series ona thin substrate, coupled to a handle substrate, in accordance with thepresent principles;

FIG. 1B is a cross-sectional view of the device of FIG. 1A furtherprocessed to form a plurality of photovoltaic cells monolithicallyintegrated and connected in series on a second side of the thinsubstrate by transferring the thin substrate to a second handlesubstrate in accordance with the present principles;

FIG. 1C is a cross-sectional view of the device of FIG. 1B with thesecond handle substrate removed in accordance with the presentprinciples;

FIG. 1D is a cross-sectional view of the device of FIG. 1A having areverse stacking of layers relative to a handle substrate in accordancewith the present principles;

FIG. 2 is a cross-sectional view of the device of FIG. 1D showingheterogeneous photovoltaic cells connected in series in accordance withthe present principles;

FIG. 3A is a cross-sectional view of a device having a plurality ofphotovoltaic cells monolithically integrated and connected in series ona SOI substrate in accordance with the present principles;

FIG. 3B is a cross-sectional view of the device of FIG. 3A wherein thesubstrate and doping layer have a dopant type having an oppositeconductivity from that of FIG. 3A in accordance with the presentprinciples;

FIG. 4 is a cross-sectional view of the device of FIG. 3B showingheterogeneous photovoltaic cells connected in series in accordance withthe present principles;

FIG. 5A is a cross-sectional view of a device having a plurality ofphotovoltaic cells monolithically integrated and connected in series ona SOI substrate with spacings etched through the dope layer to separatethe cells in accordance with the present principles;

FIG. 5B is a cross-sectional view of the device of FIG. 5A wherein thesubstrate and doping layer have a dopant type having an oppositeconductivity from that of FIG. 5A in accordance with the presentprinciples;

FIG. 6 is a block/flow diagram showing methods for forming the devicesof FIGS. 1A-1D in accordance with illustrative embodiments; and

FIG. 7 is a block/flow diagram showing methods for forming the devicesof FIGS. 3A-3B and FIGS. 5A-5B in accordance with illustrativeembodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, monolithic integration andresulting devices are provided for photovoltaic cells. In oneembodiment, heterojunction solar cells, and in particular heterojunctionwith thin intrinsic layer (HIT) cells, are integrated on thin bodysemiconductor-on-insulator substrates and/or thin layers of crystallinesemiconductors. The photovoltaic cells share at least a substratetherebetween and are connected in series to increase the output voltageof the integrated photovoltaic structure. This is because the opencircuit voltage of the integrated photovoltaic structure is, to thefirst order, equal to the sum of the open circuit voltages of theindividual solar cell units connected in series.

It is to be understood that the present invention will be described interms of a given illustrative architecture placed on a wafer; however,other architectures, structures, substrate materials and processfeatures and steps may be varied within the scope of the presentinvention. It is also to be understood that the present invention mayinclude tandem (multi-junction) structures.

It will also be further understood that when an element such as a layer,region or substrate is referred to as being “on” or “over” anotherelement, it can be directly on the other element or intervening elementsmay also be present. In contrast, when an element is referred to asbeing “directly on” or “directly over” another element, there are nointervening elements present. It will also be understood that when anelement is referred to as being “connected” or “coupled” to anotherelement, it can be directly connected or coupled to the other element orintervening elements may be present. In contrast, when an element isreferred to as being “directly connected” or “directly coupled” toanother element, there are no intervening elements present.

A design for a photovoltaic device may be created for integrated circuitintegration or may be combined with components on a printed circuitboard. The circuit/board may be embodied in a graphical computerprogramming language, and stored in a computer storage medium (such as adisk, tape, physical hard drive, or virtual hard drive such as in astorage access network). If the designer does not fabricate chips or thephotolithographic masks used to fabricate chips or photovoltaic devices,the designer may transmit the resulting design by physical means (e.g.,by providing a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication ofphotovoltaic devices and/or integrated circuit chips with photovoltaicdevices. The resulting devices/chips can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged devices/chips), as a bare die, or in a packagedform. In the latter case the device/chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case, thedevices/chips are then integrated with other chips, discrete circuitelements, and/or other signal processing devices as part of either (a)an intermediate product, such as a motherboard, or (b) an end product.The end product can be any product that includes integrated circuitchips, ranging from toys, energy collectors, solar devices and otherapplications including computer products or devices having a display, akeyboard or other input device, and a central processor. Thephotovoltaic devices described herein are particularly useful for solarcells or panels employed to provide power to electronic devices, homes,buildings, vehicles, etc.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., GaInP, InGaAs or SiGe. Thesecompounds include different proportions of the elements within thecompound, e.g., InGaAs includes In_(x), Ga_(y)As_(1-x-y), where x, y areless than or equal to 1, or SiGe includes Si_(x)Ge_(1-x) where x is lessthan or equal to 1, etc. In addition, other elements may be included inthe compound, such as, e.g., AlInGaAs, and still function in accordancewith the present principles. The compounds with additional elements willbe referred to herein as alloys.

The present embodiments may be part of a photovoltaic device or circuit,and the circuits as described herein may be part of a design for anintegrated circuit chip, a solar cell, a light sensitive device, etc.The photovoltaic device may be a large scale device on the order of feetor meters in length and/or width, or may be a small scale device for usein calculators, solar powered lights, etc.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1A, a partially fabricatedphotovoltaic device 100 is illustratively shown. In one embodiment, astack of layers 134 including a thin semiconductor layer or substrate132 is bonded to a support or handle substrate 102. A selectivelyremovable sacrificial layer 103 may be employed for transferring thesubstrate 132, if needed, as will be described. The substrate 102 isbonded to the substrate 132 and a stack of layers 134 is formed on thesubstrate 132. Substrate or layer 132 includes a p⁻ or n⁻ dopedsemiconductor material, such as Si, SiGe, Ge, SiC, III-V, etc. Substrateor layer 132 is further doped into p⁻ or n⁻ regions 112 or 114,respectively, by known doping techniques, such as, e.g., selectiveimplantation and/or diffusion (if the semiconductor is n⁻, the dopedregions are p⁻ and vice versa). This is preferably performed on thesemiconductor substrate 132 (host substrate) prior to layer transfer andlayer formation on the substrate 132 to prevent high temperature damageif other layers were present during the doping process. Isolationregions 133 may be formed by doping or by etching sections followed bydepositing a dielectric material in the regions 133.

A stack of layers 134 includes an intrinsic layer 110, which may includeamorphous hydrogenated silicon (a-Si:H), although other materials may beemployed. A doped layer 116 may include an a-Si:H (p⁺ or n⁺) layer. Atransparent conductive oxide 118 (TCO), e.g., ZnO, indium tin oxide(ITO), etc. is deposited on doped layer 116. The TCO 118 is patterned byperforming an isolation etch to form TCO islands 118.

Note that instead of depositing and patterning the TCO 118, patternedTCO may be deposited by using shadow masks. In some embodiments, theisolation etch is performed on the host substrate 132. (i.e., prior tolayer transfer). In some embodiments, isolation is performed bytechniques other than etching, for example by selective (i.e. masked)implantation to locally amorphize sections of the TCO islands 118 andtherefore reduce the conductivity or by local oxidation/nitridation forexample by local laser radiation or implantation of oxygen or nitrogen.These techniques are preferably performed on the host substrate 132prior to layer transfer.

Low-temperature bonding may be employed after layers are formed on thesubstrate 132 as will be described. However, the stack of layers 134 asshown in FIG. 1A may be employed as monofacial device with multiplephotovoltaic cells 130. The cells 130 are connected in series andinclude TCO islands aligned with the alternating doped regions 112 and114. It should be understood that the handle or holding substrate 102may remain as part of the device 100 or be removed and the substrate 132transferred to another platform or substrate.

Referring to FIG. 1B, the stack of layers 134 and substrate 132 aretransferred to another platform or substrate. This may be to provideadditional cell junctions and/or electrodes on an opposite face or sideof the substrate 132 or to transfer the device 100 to another device orplatform. In one embodiment, the device 100 is bonded to a second handleor holding substrate 105 on an opposite side from the handle substrate102. Then, the substrate 102 is debonded. In one embodiment, this mayinclude removing the sacrificial layer 103 by and etch or other process.It should also be understood that wafer bonding and removal may includeany number of processes in addition to those described herein. Forexample, spalling techniques, adhesive bonding and release agents, etc.may be employed.

Now, a second side of the substrate 132 is exposed and may be employedfor formation of additional layers to complete a device 101. A stack oflayers 136 includes an intrinsic layer 108, a doped layer 106 (n⁺ orp⁺). The doping of layer 106 has opposite conductivity relative to layer116. For example, if layer 106 is p⁺ doped, layer 116 is n⁺doped. Thelayers 106 and 108 may include a-Si:H although other suitable materialsmay be employed. A conductive material (e.g., TCO) 119 is deposited onthe other side of the substrate 132, followed by patterning of thematerial to form islands 119 using one of the techniques describedabove. This forms a plurality of adjacent solar cells 130 monolithicallyintegrated in a same substrate.

Since the a-Si:H layers (106, 108, 110, 116) are very thin and theconductivity of the doped a-Si:H layers 106, 116 is generally low (e.g.,conductivity of lower than 0.01 (Ω·cm)⁻¹, i.e., sheet resistance of ˜100MΩ/Sq. or higher for layers of ˜10 nm or thinner), lateral conduction ofthese layers 106, 116 is negligible. Therefore, effectively, theadjacent solar cells 130 are laterally isolated and connected in seriesonly through electrical connections made by the TCO layers 118 and 119.In some embodiments, the patterned TCO 118, 119 may be used as a mask toremove the a-Si:H layers 106, 108, 110, 116 in between the two adjacentcells 130. Note than the TCO layers 118 or 119 on one side of thesubstrate may be replaced by metal layers as transparency is needed onlyon one side of the substrate 132 (to allow light to enter the solarcell). However, TCO 118, 119 may be used on both sides for bifacialoperation (to allow light to enter from both sides). This is shown inone embodiment in FIG. 1C after the handle substrate 105 has beendebonded. It should be understood that the substrate 105 may remain inplace in the finished device, and may be transparent for receiving lighttherethrough (for monofacial or bifacial operation). Alternately, thesubstrate 105 may be removed as well using, e.g., a sacrificial layer.

FIG. 1C shows an illustrative device 150 having no handle substrate. Thedevice 150 may provide monofacial or bifacial operation. The device maybe transferred to a printed wiring board, substrate or other surface ordevice using the transfer processes described above where light can bereceived from both a top and bottom.

In some embodiments, as in FIG. 1B, the support substrates 102 or 105may be permanent. Before bonding the stack of layers 134 to the supportsubstrate 105, a layer of reflective metal (such as silver) may beformed on the back of the TCO layer 118 (or TCO layers 119) on one side(for example after TCO deposition followed by simultaneous patterning ofthe TCO and the reflective metal, or by using the same shadow mask todeposit the patterned TCO and metal) to increase the reflection of lightback into the solar cell for better light trapping (a back reflector).Since the heterojunction solar cells 130 are connected in series, theoutput voltage of the integrated unit is increased accordingly. If thehandle substrate 105 (or 102) is flexible, the integrated unit (100,101) may be used as a flexible power supply in applications that requireflexibility such as wearable electronics or flexible displays.

FIG. 1D shows a device 152 where light can be received through thesubstrate 102 which is employed as a permanent part of the device. Thesubstrate 102 is transparent and may be formed from glass, polymer orother transparent material and permit bifacial operation.

Referring to FIG. 2, the device 152 of FIG. 1D is illustrativelydepicted to show series connections between adjacent cells 140 and 142.In addition, materials employed are provided to illustrate the structurein accordance with one embodiment, other materials and structures mayalso be employed. A first heterjunction cell 140 includes an emitterportion 144 and a back-surface-field (BSF) contact portion 145. In thisexample, the emitter portion 144 of the cell 140 includes TCO 118,n+a-Si:H layer 106, and intrinsic a-Si:H layer 108. The BSF portion 145of the cell 140 includes p+a-Si:H layer 110, intrinsic a-Si:H layer 116and TCO 119. A p− c-Si substrate 112 is disposed between the emitterportion 144 and the BSF portion 145.

A second heterojunction cell 142 (adjacent to the first cell 140 andconnected in series through the TCO 118 and 119) includes an emitterportion 147 and a BSF contact portion 146. In this example, the emitterportion 147 of the cell 142 includes p+a-Si:H layer 110, intrinsica-Si:H layer 116 and TCO 119. The BSF portion 146 of the cell 142includes TCO 118, n+a-Si:H layer 106, and intrinsic a-Si:H layer 108. Ann− c-Si substrate 114 is disposed between the emitter portion 147 andthe BSF portion 146.

In FIG. 2, lines 154 illustratively show the flow of current betweenadjacent cells 140 and 142. The photovoltaic cells 140 and 142 share atleast the substrate layer 132 therebetween and are connected in seriesthrough TCO 118, 119 to increase the output voltage of the integratedphotovoltaic structure. The open circuit voltage of the integratedphotovoltaic structure is, to the first order, equal to the sum of theopen circuit voltages of the individual solar cell units connected inseries. Note that similar operation and structure is provided by thestructures of FIGS. 1B and 1C.

Referring to FIGS. 3A and 3B, another exemplary integration scheme isillustratively shown that employs a semiconductor-on-insulator (SOI)substrate 202. The SOI substrate 202 includes a base substrate 204 andburied dielectric layer 206 and a thin crystalline layer or substratelayer 208. The substrate layer 208 is doped by known methods to formhigh-low doped regions 210, 212, which respectively include p⁺ and p⁻doped regions in FIG. 3A, and high-low doped regions 214, 216, whichrespectively include n⁺ and n⁻ doped regions in FIG. 3B. The dopedregions 210, 212, 214, 216 may be formed by, for example, maskedimplantation. The p− (212) and p+ (210) regions alternate with the p+regions preferably being smaller, and the n− (216) and n+ (214) regionsalternate with the n+ regions preferably being smaller. Isolationregions 218 are also formed by, e.g., etching or other techniques knownin the art. Next, an intrinsic layer 220 is formed on the substrate 208.An n⁺ doped layer 222 (FIG. 2A) or a p⁺ doped layer 224 (FIG. 2B) isdeposited on the intrinsic layer 220. The intrinsic layer 220 and thedoped layers 222, 224 may include a-Si:H, although other suitablematerials may be employed. A TCO stack is deposited and etched to formTCO islands 226 as described above.

Due to the negligible lateral conduction of the doped layers 222 and224, heterojunction solar cells 230 are integrated in series. In someembodiments, the patterned TCO 226 may be used as a mask to createspacings between portions of the layers 220, 222, 224. This structure isillustratively depicted in FIGS. 5A and 5B where the layers in betweentwo adjacent cells 330 are etched. Note that the doped layers 222, and224 form a tunnel junction with the doped regions 210 and 214,respectively, due to the thin intrinsic layers 220 and to the firstorder may be approximated as an ohmic contact (in the verticaldirection). The thickness of the intrinsic layer (108, 110, 220) in theembodiments of FIGS. 1A-1D and FIGS. 3A-3B is preferably below about 10nm, although thicker layers may be employed. The thickness of the dopedlayers (106, 116, 222, 224) is preferably in the range of about 3-20 nm,although thinner or thicker layers may be employed. Note that theintrinsic layer (108, 110, 220) may be omitted in some embodiments.

Referring to FIG. 4, the device of FIG. 3B is illustratively depicted toshow series connections between heterojunction cells 240 and tunneljunctions 242. In addition, materials employed are provided toillustrate the structure in accordance with one embodiment, othermaterials and structures may also be employed. A heterojunction cell 240includes TCO 226, n+ a-Si:H layer 222, intrinsic a-Si:H layer 220, a p−c-Si substrate 212 and a p+ c-Si substrate 210. Tunnel junction 242includes n+ a-Si:H layer 222, intrinsic a-Si:H layer 220 and a p+ c-Sisubstrate 210. Note that similar operation and structure is provided bythe structure of FIG. 3B.

In FIG. 4, lines 244 illustratively show the flow of current betweencell 240 and junction 242. The cell 240 and junction 242 share at leastthe substrate layer 208 therebetween and are connected in series throughTCO 226 to increase the output voltage of the integrated photovoltaicstructure. The open circuit voltage of the integrated photovoltaicstructure is, to the first order, equal to the sum of the open circuitvoltages of the individual solar cell units connected in series.

Referring to FIGS. 5A and 5B, in some embodiments, the intrinsic layers(108, 110 (FIG. 1B, FIG. 1C), 220 (FIG. 3A, 3B) of front and/or backstacks may be replaced by a doped hydrogenated crystalline material(e.g., n⁺ c-Si:H 240 or p⁺ c-Si:H 241 in FIGS. 5A and 5B, respectively).However, since the lateral conductively is greater in such cases etchingthrough the doped layers is needed to create spacings 302. This is shownschematically in FIGS. 5A and 5B for the structures of FIGS. 3A and 3B;however, it is applicable to either or both of the front and backcontacts in FIGS. 1B, 1C and/or 1D. The spacings 302 are preferablycreated by using a patterned TCO 226 as a mask and etching the layersdown to the substrate 208. In other embodiments, the stack may includelayer 240 or 241 and/or an intrinsic layer or a doped layer (the sameconductivity as layer 240 or 241 which it is in contact with). In theseembodiments, the thickness of the doped c-Si:H layers 240, 241 is in therange of about 2-30 nm, although thinner or thicker layers may be usedas well.

The hydrogenated crystalline material may be single crystalline,polycrystalline, or combinations thereof. For example, if the substrate208 is mono-crystalline, the hydrogenated crystalline layers 240 and 241are single-crystalline, and if the substrate 208 is multi-crystalline,the hydrogenated crystalline layers 240 and 241 are poly-crystalline. Inpreferred embodiments, the substrate 208 is mono-crystalline, and thehydrogenated crystalline layers 240 and 241 are single-crystalline. Thedoped hydrogenated crystalline layer may include hydrogen content in therange of 5-40 atomic percent. The doped hydrogenated crystalline layermay include an activated doping concentration higher than 5×10¹⁸ cm⁻³.

For all embodiments, substrate materials (base substrate, thincrystalline substrate layer, etc.) preferably include crystallinematerials. Crystalline substrate or crystalline materials refer tosingle-crystalline or multi-polycrystalline substrates or materials.Other group IV semiconductor materials such as germanium (Ge),silicon-germanium (SiGe), silicon carbide (SiC) and silicon germaniumcarbide (SiGeC) may also be employed. In addition, group III-Vsemiconductor materials may be used as well. Examples include but arenot limited to gallium nitride (GaN), gallium phosphorus nitride (GaPN),aluminum antimonide (AlSb), aluminum arsenide (AlAs), aluminum nitride(AlN), aluminum phosphide (AlP), boron nitride (BN), boron phosphide(BP), boron arsenide (BAs), gallium arsenide (GaAs), gallium phosphide(GaP), indium nitride (InN), indium phosphide (InP), aluminum galliumarsenide (AlGaAs), indium gallium phosphide (InGaP), aluminum indiumarsenic (AlInAs), aluminum indium antimonide (AlInSb), gallium arsenidenitride (GaAsN), gallium arsenide antimonide (GaAsSb), aluminum galliumnitride (AlGaN), aluminum gallium phosphide (AlGaP), indium galliumnitride (InGaN), indium arsenide antimonide (InAsSb), indium galliumantimonide (InGaSb), aluminum gallium indium phosphide (AlGaInP),aluminum gallium arsenide phosphide (AlGaAsP), indium gallium arsenidephosphide (InGaAsP), indium arsenide antimonide phosphide (InArSbP),aluminum indium arsenide phosphide (AlInAsP), aluminum gallium arsenidenitride (AlGaAsN), indium gallium arsenide nitride (InGaAsN), indiumaluminum arsenide nitride (InAlAsN), gallium arsenide antimonide nitride(GaAsSbN), gallium indium nitride arsenide aluminum antimonide(GaInNAsSb), gallium indium arsenide antimonide phosphide (GaInAsSbP),and combinations thereof.

In the embodiments described above, a-Si:H has been used as an exampleof non-crystalline semiconductor material employed for intrinsic and/ordoped layers. However, other materials such as hydrogenated amorphousgermanium (a-Ge:H), hydrogenated amorphous silicon carbide (a-SiC:H),hydrogenated amorphous silicon germanium (a-SiGe:H), or combinationsthereof may be used as well. In addition to or instead of amorphousforms of Si, Ge, SiC, SiGe, etc., the nanocrystalline ormicrocrystalline forms of these materials or combinations thereof mayalso be employed.

The amorphous, nanocrystalline, microcrystalline and crystalline formsof the described layers may contain nitrogen (N), oxygen (O), fluorine(F), deuterium (D), chlorine (Cl) or combinations thereof. Theamorphous, nanocrystalline, microcrystalline materials may be grown byplasma-enhanced chemical vapor deposition (PECVD), however, other growthtechniques known in the art, such as hot-wire CVD (HWCVD) or sputteringmay be employed as well. In the case of PECVD or HWCVD, the gas sourceused for growing Si containing layers is silane (SiH₄) although othergases such as disilane (Si₂H₆), dichlorosilane (DCS), tetrafluorosilane(SiF₄) or combinations thereof may be used as well. These gas sourcesmay be mixed with hydrogen during growth. The gas source used for Gecontaining layers may include germane (GeH₄). The gas source used for Ccontaining layers may include methane (CH₄), ethylene (C₂H₄), propylene(C₃H₆) but other sources (e.g., of the form C_(x)H_(y)) may also beemployed. Ammonia (NH₃), nitrous oxide (N₂O) or other gas sources may beused for nitrogen containing layers. Carbon dioxide (CO₂), N₂O or O₂ maybe used to provide oxygen for oxygen containing layers. A carrier gassuch as hydrogen (H₂), deuterium (D₂) helium (He) or argon (Ar) may beused for formation of any or all of the layers. The carrier gas may bepre-mixed with the gas sources or flowed simultaneously with the gassource at the time of growth.

In-situ p-type doping may be performed using diborane (B₂H₆) ortrimethylboron (TMB) sources, and in-situ n-type doping may be performedusing a phosphine (PH₃) gas source, although other dopant sources mayalso be employed.

Hydrogenated crystalline silicon (c-Si:H) has been used as an example ofthe highly-doped hydrogenated crystalline material. However, c-SiGe:Hand c-Ge:H may be employed as well. The gas sources used for the growthof the hydrogenated crystalline Si, Ge (and SiGe) may include but arenot limited to SiH₄ (silane), GeH₄ (germane) (and combinations thereof)and H₂. Other examples of Si containing gas sources are Si₂H₆ and SiF₄.For SiGe layers, a gas flow ratio [H₂]/([SiH₄]+[GeH₄])>5 is preferable.Dopant gases (flowed in-situ) may include but are not limited to PH₃(phosphine) for n-type doping, and B₂H₆ (diborane) or TMB(tri-methyl-boron) for p-type doping. These layers may contain nitrogen(N), oxygen (O), fluorine (F), deuterium (D), chlorine (Cl) orcombinations thereof. The preferred technique for growing thesematerials is PECVD, however, other growth methods such as hot wire CVD(HWCVD) may be employed. The PECVD tool may use RF or microwave powerand may or may not have a remote-plasma configuration.

Growth temperatures for the above processes are preferably close to 200°C., with highest quality films being grown at temperatures in the rangeof 150-250° C., however growth temperatures in the range ofroom-temperature to 450° C. may also be employed. The hydrogen contentof the hydrogenated crystalline material is in the range of 5-40 atomicpercent. The growth of the hydrogenated crystalline materials may becarried out as described in U.S. patent application Ser. No. 13/032,866filed on Feb. 23, 2011, and incorporated herein as a reference.

Referring again to FIGS. 1A-1D and FIGS. 3A-3B, with respect to“contacts” formed by the stacks of layers referred to in FIGS. 1A-1D andFIGS. 3A-3B, the intrinsic layers, e.g., 108, 110, 220 may each be asingle layer or a multilayer. The composition of the intrinsic layer maybe constant or vary across the layer. In some embodiments, the dopedlayers (e.g., layers 106, 116, 222, 224) are comprised of a bilayer,wherein a second layer has a bandgap lower than that of a first layer ofthe bilayer (where the first layer is the layer which is in directcontact with the intrinsic layer). In these embodiments, the secondlayer which has a lower bandgap may have a higher doping activation thanthe first layer, and provide better majority carrier transport whilebetter repelling the minority carriers from the contact.

In some embodiments, where the doped layer is a bilayer, the doping ofthe doped layer is enhanced by modulation doping (or transfer doping),i.e. transfer of carriers of the corresponding conductivity type fromthe material with a larger bandgap into the material with the lowerbandgap, or the transfer of the opposite type of carriers from thematerial with the lower bandgap into the material with a higher bandgap.The carriers corresponding to n-type conductivity are electrons, and thecarriers corresponding to p-type conductivity are holes. In someembodiments, the doped layer is comprised of a superlattice, i.e., amulti-layer comprised of alternating materials with low and highbandgaps (with respect to each other), wherein the doping of the narrowgap layers is enhanced by transfer doping. The superlattice structuresmay be formed by selecting respectively high and low bandgap materialsfrom a group of materials, such as, e.g., a-Si:H, a-Ge:H, a-SiGe:H,a-SiC:H, a-SiO:H, a-SiN:H, nc-Si:H, nc-Ge:H, nc-SiGe:H, nc-SiC:H,nc-SiO:H, nc-SiN:H, pc-Si:H, pc-Ge:H, pc-SiGe:H, pc-SiC:H, pc-SiO:H,pc-SiN:H or combinations thereof.

The bandgap of a-Si:H is typically in the range of 1.7-1.8 eV; however,as known in the art, larger or smaller bandgaps are possible by varyingthe deposition conditions. The bandgap of a-Ge:H is in the range of0.9-1.2 eV; however, similar to a-Si:H, larger and smaller bandgaps arealso possible. An alloy of two semiconductor materials has a bandgapdepending linearly on the atomic fractions of the two semiconductors,e.g., an alloy of a-Si_(x)Ge_(1-x) has a bandgap of xE_(g1)+(1−x)E_(g2),where E_(g1) is the bandgap of a-Si:H, E_(g2) is the bandgap of a-Ge:H,x is the atomic fraction of Si (number of Si atoms in the latticedivided by the total number of Si and Ge atoms), and 1-x is the atomicfraction of Ge. (Note that 0≦x≦1). Similarly, the bandgap of ana-Si_(x)C_(1-x):H alloy is increased from 1.7-1.8 eV to 3.6-3.7 eV asthe atomic fraction of C is increased from 0 to 1. Addition of N, O orboth to a-Si:H, a-Ge:H or a-SiGe:H increases the bandgap, but theincrease in bandgap is not typically a linear function of the atomicpercentage. Hydrogenated amorphous silicon-nitride (a-Si_(x)N_(1-x):H)has a bandgap of 5-5.5 eV for the stoichiometric composition x=0.42, andthe bandgap can be varied by changing the atomic percentage of N.Hydrogenated amorphous silicon oxide (a-SiO:H) has bandgaps comparablewith a-SiN:H at comparable atomic fractions of Si.

The bandgap of semiconductor alloys or oxides/nitrides may be varied bychanging the crystalline portion of the materials. The bandgap ofnanocrystalline or microcrystalline materials is generally lower thanthat of amorphous materials having the same composition. The bandgap ofhydrogenated nano/microcrystalline Si varies between ˜1.1 eV to ˜1.8 eVas the material structure varies from fully single/poly-crystalline tofully amorphous. Similarly, the bandgap of hydrogenatednano/microcrystalline Ge varies between ˜0.6 eV to ˜1.2 eV as thematerial structure varies from fully single/poly-crystalline to fullyamorphous. Similarly, the bandgaps of hydrogenatednano/micro-crystalline SiC, SiO, SiGeO, GeO and SiN compounds are lowerthan that of a-SiC:H, a-SiO:H, a-SiGeO:H, a-GeO:H and a-SiN:H,respectively. This also applies to combinations of these materials. Inthe PECVD process, the crystalline portion of the materials may beincreased by increasing the hydrogen dilution of the source gases,increasing the plasma frequency (e.g., up to 120 MHz) or both. Forexample, hydrogenated nano-crystalline silicon oxide (nc-SiO:H) may begrown by PECVD with a bandgap in the range of 0.8-2.5 eV, depending onthe growth conditions and oxygen content of the film.

Referring to FIGS. 6 and 7, flowcharts/block diagrams in FIGS. 6 and 7illustrate the architecture, functionality, and operation of possibleimplementations according to various embodiments of the presentinvention. It should also be noted that, in some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

Referring to FIG. 6, a method for fabricating a device with integratedphotovoltaic cells is illustratively shown. In block 402, asemiconductor substrate is supported on a first handle substrate. Asacrificial layer or layers may be employed between these substrates toselectively debond the substrates during a transfer process.

In block 404, the semiconductor substrate is doped to form alternatingdoped regions with opposite conductivity. Isolation regions may also beformed. In block 406, an intrinsic layer may be formed on thesemiconductor substrate. In block 410, at least one doped layer isformed over a first side the semiconductor substrate. In one embodiment,forming at least one doped layer includes forming a continuous dopedlayer through the plurality of cells. The at least one doped layerincludes a doped non-crystalline form of a semiconductor material, whichprovides a high lateral resistive path to prevent lateral conductionbetween the plurality of cells.

In block 412, a conductive material is patterned over the least onedoped layer to form conductive islands such that the conductive islandsare aligned with the alternating doped regions on the substrate todefine a plurality of photovoltaic cells connected in series on amonolithic structure. The conductive material of the conductive islandsmay include a transparent conductive oxide.

In block 414, the monolithic structure may be transferred to a secondhandle substrate. In block 416, the monolithic structure is debondedfrom the first handle substrate to expose a second side of thesemiconductor substrate or to complete the device. This may include anetch process of a sacrificial layer or the like.

In block 418, an intrinsic layer may be formed on the semiconductorsubstrate. In block 420, at least one other doped layer is formed overthe second side the semiconductor substrate. In block 422, another layerof conductive material is patterned over the least one other doped layerto form conductive islands such that the conductive islands are alignedwith the alternating doped regions of the substrate to further definephotovoltaic cells connected in series on the monolithic structure. Thesecond handle substrate may be transparent and remain on the monolithicstructure to form a bifacial device. The second handle substrate mayalso be removed to form a bifacial device.

In another embodiment, in block 424, an etch of the at least one dopedlayer may be performed using the conductive islands as an etch mask orotherwise to break up the doped layer or layers to prevent lateralconduction between cells through the doped layer or layers. This mayinclude providing doped layers instead of the intrinsic layers toprovide a conductive path. In block 426, processing continues tocomplete the device.

Referring to FIG. 5, another method for fabricating a device withintegrated photovoltaic cells is shown. In block 502, a semiconductorlayer on a buried dielectric layer of a semiconductor-on-insulatorsubstrate is doped to form alternating doped regions of higher and lowerdopant concentration across the semiconductor layer. Isolation regionsmay also be formed. In block 504, an intrinsic layer may be formedbetween the at least one doped layer and the semiconductor layer.

In block 506, at least one doped layer is formed over a first side thesemiconductor layer. In one embodiment, a continuous doped layer isformed through the plurality of cells, and the at least one doped layerincludes a doped non-crystalline form of a semiconductor material toprevent lateral conduction between the plurality of cells.

In block 512, a conductive material is patterned over the least onedoped layer to form conductive islands such that the conductive islandsare aligned with the alternating doped regions to define a plurality ofphotovoltaic cells connected in series on a monolithic structure. Theconductive material for the conductive islands may include a transparentconductive oxide. In block 514, in another embodiment, the at least onedoped layer includes a crystalline semiconductor, and the at least onedoped layer is etched using the conductive islands as an etch mask. Thisprevents lateral conduction when conductive materials are employed forthe doped layers. This may include providing doped layers instead of theintrinsic layers to provide a conductive path. In block 516, processingcontinues to complete the device.

Having described preferred embodiments for monolithic integration ofheterojunction solar cells (which are intended to be illustrative andnot limiting), it is noted that modifications and variations can be madeby persons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A device with integrated photovoltaic cells,comprising: a semiconductor layer having alternating regions of higherand lower dopant concentration of a same dopant type across thesemiconductor layer; at least one doped layer formed over a first sideof the semiconductor layer; and a patterned conductive material formedover the least one doped layer forming conductive islands such that theconductive islands are aligned with the alternating regions to define aplurality of photovoltaic cells.
 2. The device as recited in claim 1,further comprising an intrinsic layer formed between the at least onedoped layer and the semiconductor layer.
 3. The device as recited inclaim 1, further comprising a doped hydrogenated crystalline layerformed between the at least one doped layer and the semiconductor layerand having a same conductivity as the at least one doped layer.
 4. Thedevice as recited in claim 3, wherein the doped hydrogenated crystallinelayer is formed in direct contact with the semiconductor layer.
 5. Thedevice as recited in claim 1, wherein the conductive material includes atransparent conductive oxide.
 6. The device as recited in claim 1,wherein the at least one doped layer includes a continuous doped layerformed through the plurality of cells, the at least one doped layerincludes a doped non-crystalline form of a semiconductor material toprevent lateral conduction between the plurality of cells.
 7. The deviceas recited in claim 6, further comprising spacings formed through the atleast one doped layer corresponding with the conductive islands.
 8. Thedevice as recited in claim 7, wherein the spacings are formed through adoped hydrogenated crystalline layer, which is formed between the atleast one doped layer and the semiconductor layer, the dopedhydrogenated crystalline layer having a same conductivity as the atleast one doped layer.
 9. The device as recited in claim 1, furthercomprising non-doped regions in the semiconductor layer disposed betweeneach of the plurality of photovoltaic cells.
 10. The device as recitedin claim 9, wherein each of the conductive islands is disposed over atleast one higher dopant concentration, at least one non-doped region andat least one lower dopant concentration.
 11. The device as recited inclaim 1, wherein the at least one doped layer includes a bilayer havinga first layer and a second layer, the first layer having a bandgap lowerthan that of the second layer.
 12. The device of claim 11, wherein thesecond layer is disposed between the first layer and the semiconductorlayer.
 13. The device as recited in claim 1, wherein each of thealternating regions extends within an entire thickness of thesemiconductor layer.